Electronic ballasts for powering a gas discharge lamp must provide a high power factor, low total harmonic distortion, and a high efficiency. Simultaneously, the ballasts must be economical to manufacture. Additionally, the ballast must start the lamp quickly.
Electronic ballasts convert AC (alternating current) power at a relatively low frequency (approximately 60 Hz) to AC power at a relatively high frequency. In order to attain good lamp life, the AC power supplied to the gas discharge lamp should be sinusoidal.
A resonant circuit is used to attain the sinusoidal power for the lamps. Two types of resonant circuits are the series resonant circuit and the parallel resonant circuit. A series resonant circuit has an inductor in series with a capacitor. A parallel resonant circuit has an inductor in parallel with a capacitor. The inductance of the inductor and the capacitance of the capacitor are selected so that the resonant circuit resonates at a select, high frequency, usually on the order of 30 KHz (kilohertz).
Ballasts with parallel resonant circuits are generally less expensive to build than ballasts with series resonant circuits. However, parallel resonant circuits are less efficient than the series resonant circuits, due to high losses if the voltage across the parallel resonant circuit is low.
Therefore, a ballast using a parallel resonant circuit which attains a high power factor and low total harmonic distortion is desirable.